Modified atmosphere dielectric barrier discharge (MADBD) treatment has been used for many years for the surface treatment of polymeric substrates. U.S. Pat. No. 7,147,758 for example is concerned with such treatments in the presence of a carrier gas, a reducing gas and an oxidising gas. It is not uncommon in the art for MADBD treatment to be called plasma treatment. In this specification no distinction is made between plasma treatment on the one hand and MADBD treatment on the other. However, both are treatments which typically take place in a modified gas atmosphere (i.e. an atmosphere other than air). Corona discharge treatment (also known as corona treatment or D treatment), is another form of dielectric barrier discharge which typically takes place at lower power (and with a larger electrode gap) than MADBD or plasma treatment, and typically takes place in an unmodified atmosphere—i.e. air.
Corona discharge treatment has been used considerably longer than MADBD treatment in the processing of polymeric films, and is an established technique in the industry. However, typically the manufacturers of modified atmosphere MADBD treaters have cautioned against using corona treatment in combination with MADBD treatment, apparently believing that the surface chemistry of MADBD treated film would be adversely affected by corona treatment. Consequently, it has rarely been contemplated to subject film to both MADBD and corona discharge treatment. U.S. Pat. No. 5,147,678 appears to contemplate such combinative treatments, but only in the context of laboratory experimentation and with unproven commercial utility. U.S. Pat. No. 7,824,600 expressly contemplates a two stage treatment in which a monoaxially oriented film is subjected to a plasma treatment before being laterally stretched and corona treated prior to winding onto a reel. This document fails to appreciate that any benefit may be derived from a further or alternative downstream treatment of the film, and instead concentrates only on multiple treatments taking place before winding of the film onto a reel. On the other hand the benefit of downstream treatment is apparently recognised in U.S. Pat. No. 7,410,675, but only in the context of a repetition of a treatment having already once been conducted on the film.
One problem with MADBD treatment is that whatever surface modification of the film takes place under such treatment, the effect is not permanent, so that a treated film with surface characteristics making it suitable for printing tends to lose those characteristics over time and revert to being unprintable or poorly printable. This causes serious problems in the film industry because film manufacturers are rarely responsible for printing the films they make. Commonly, film manufacturers will instead wind film onto a reel and ship it to their customers, typically printers or converters, who will unwind the film prior to converting and/or printing it. Inevitably in connection with a MADBD treated film, by the time the film is then printed much of the surface characterisation caused by the MADBD treatment has been lost. Hitherto, film manufacturers have consequently sought to guarantee the long-term printability of the film by means other than MADBD treatment—the provision of printable coatings on the film for example.
What has now been realised is that the surface characterisation of the film caused by MADBD treatment can be revived, improved or reconstituted considerably after (even many months after) initial manufacture and MADBD treatment of the film by the apparently straightforward expedient of corona treating the previously MADBD treated film. The combination of an initial MADBD treatment (normally during manufacture of the film) and a downstream corona treatment to refresh or even augment the surface properties of the MADBD-treated film has not hitherto been recognised in the art. Other combinatory and/or repetitious treatments mentioned in the art which also fail to appreciate this concept are disclosed in EP0947544, U.S. Pat. Nos. 7,300,859, 7,067,405, WO2008102408,U.S. Pat. No. 4,929,319, EP1620262, JP11256338 and JP9314773.